Romney to CPAC: 'Learn from My Mistakes'

Romney also lauded "the clear and convincing voice of my friend, Paul Ryan."

"If I were to offer advice to any president of the United States, it would be this: do whatever you can do to keep America the most prosperous and free and powerful nation on earth," he said, warning of the entities jockeying for world leadership: "China, Russia, and the jihadists."

He lauded America's role in world, including never using "hegemonic military power" to prey upon the weak, extending philanthropy to victims of natural disasters and AIDS victims in Africa, and intervening in conflicts with the goal of liberation.

"Freedom flows in American veins. It invigorates our many enterprises, it inspires us to live beyond ourselves, it calls us to care for the suffering and downtrodden. It has made us a great nation. Today, history and duty summon us again," Romney said. "Each of us in our own way will have to step up and meet our responsibility. I am sorry that I will not be your president – but I will be your co-worker and I will stand shoulder to shoulder with you."

Romney's former running mate, Ryan (R-Wis.), spoke earlier in the day and similarly received a warm welcome from the crowd.

But Ryan conveyed the impression that he's left the veep nod behind him and is once again immersed in his role as House Budget Committee chairman. He made zero mention of the campaign and gave no indication of future political goals.

"Hey, I'm so happy to be here. You know, we all need a break from the mess in Washington," Ryan said. "And I've just got to say, it is nice to be in a room full of conservatives for a change."